Recently, a liquid crystal display device has been widely used as a display device of an information device such as a notebook-type personal computer, a word processor and the like, or as a display device of a video device such as a portable television, a video movie, a car navigation system and the like, by taking advantage of a characteristic in which the liquid crystal display device is light and thin, and consumes small electricity. Such liquid crystal display device typically has a structure in which a display element is illuminated from behind by a built-in lighting unit for obtaining a bright display screen.
As a lighting unit, there is an edge light type lighting unit in which a linear light source such as a fluorescent discharge tube is disposed on an end face of a light guiding plate disposed on a rear surface of the display element (an opposite surface of a display surface of the element). The edge light type is characterized in that a thin lighting unit and a highly uniform luminance of a light emitting surface thereof can be obtained. Therefore, in order to give priority to thinness of the lighting unit, the edge light type is commonly adopted in the lighting unit used as a back light of the liquid crystal display device composing the notebook-type personal computer and the like. In the edge light type, however, there is a proportional relationship between thickness of the entire lighting unit and the luminance thereof. So, when the luminance has priority over the thinness, the thickness increases, while when the thinness has priority over the luminance, the luminance decreases. Thus, the thinness and the luminance have a trade-off relationship, and therefore, it is necessary to solve this essential problem in order for the thinness and the luminance to be compatible with each other. Accordingly, in the liquid crystal display device used in the video movie, the car navigation system and the like, for example, the lighting unit of the edge light type in which two or more fluorescent discharge tubes are provided, or that of the edge light type in which an L-shaped or U-shaped fluorescent discharge tube is disposed along the end face of the light guiding plate is used to allow the thinness and the luminance to be compatible with each other. However, since the liquid crystal display is required to provide improved portability and to save a space, it is desirable to further reduce the thickness of the entire lighting unit while maintaining the luminance thereof.
On the other hand, when the lighting unit is operating, a high-frequency alternating current of 40 to 100 kHz is generally applied to the fluorescent discharge tube. Thereby, the fluorescent discharge tube is driven to emit light. Herein, the fluorescent discharge tube to which such high voltage is applied generates an electromagnetic wave while emitting light. It is highly possible that the electromagnetic wave radiated from the fluorescent discharge tube affects a liquid crystal display element and a circuit board disposed on a rear surface side of the lighting unit, thereby causing a display defect such as occurrence of noise or Moire fringes on a display screen. Therefore, in general, an electrical conductor such as metal covers a periphery of the fluorescent discharge tube to shield the tube.
FIG. 5 is a cross-sectional view schematically showing a structure of the liquid crystal display device comprising the lighting unit of the conventional edge light type. A liquid crystal display device LD comprises a lighting unit UT configured to illuminate a liquid crystal panel 6 from behind. The lighting unit UT comprises a light guiding plate 1 for guiding light emitted from a fluorescent discharge tube 2 to the liquid crystal panel 6, fluorescent discharge tubes 2 as light sources disposed on end faces of the light guiding plate 1, a reflecting sheet 3, and electrically conductive sheets 5. The liquid crystal panel 6 is disposed on a light emanating surface side of the lighting unit UT, while a circuit board 8 provided with a drive circuit of the lighting unit UT and the liquid crystal panel 6 is disposed on the rear surface side thereof.
Light leaking out of a bottom surface and the end faces of the light guiding plate 1 is reflected by the reflecting sheet 3 and is returned into the light guiding plate 1, thereby enabling an amount of light emitted from the light emanating surface of the light guiding plate 1 to increase. As the reflecting sheet 3, a white resinous film having a high reflectivity is used, for example. The reflecting sheet 3 is disposed so as to cover the entire bottom surface of the light guiding plate 1, and is bent in U-shape so as to enclose each of end face regions of the light guiding plate 1 and each of the fluorescent discharge tubes 2. As used herein, reflector portions RF refer to portions including the end face regions of the light guiding plate 1 and the fluorescent discharge tubes 2. The reflecting sheet 3 is bonded and fixed to the bottom surface of the light guiding plate 1 by an adhesive (not shown) such as a double face adhesive tape, and is also bonded to a front surface of the light guiding plate 1 corresponding to the reflector portion RF by an adhesive 7a. The reflecting sheet 3 may be provided with, for example, a printed pattern on a surface thereof for promoting diffusion of light as it is distant from the fluorescent discharge tube 2.
Herein, a case where the reflecting sheet 3 continuously covering the reflector portions RF and the bottom surface of the light guiding plate 1 is used is described with reference to FIG. 5. As an alternative example of the reflecting sheet 3, as shown in FIG. 6, the sheet may be structured such that sheets 3a covering the reflector portions RF and a sheet 3b covering the bottom surface of the light guiding plate 1 are provided separately, and are bonded to each other by an adhesive 7b such as the double face adhesive tape. By using the integral-type reflecting sheet 3 as shown in FIG. 5, a thin lighting unit UT, a cost reduction thereof, and a reduction of the number of assembly processes thereof are favorably realized.
As shown in FIG. 5, in order to inhibit the electromagnetic wave radiated from the fluorescent discharge tube 2 when emitting light from adversely affecting the liquid crystal panel 6 or the circuit board 8, the electrically conductive sheet 5 as an electromagnetic wave blocking component is disposed on the surface of the reflecting sheet 3 corresponding to the reflector portion RF such that the electrically conductive sheet 5 is bent in U-shape so as to enclose the reflector portions RF. The electrically conductive sheet 5 is a sheet made of an electrically conductive material such as copper foil or aluminum foil, and is bonded to the surface of the reflecting sheet 3 of the reflector portions RF by means of an adhesive applied to the surface of the sheet 5.
In the lighting unit UT and the liquid crystal display device LD thus structured, thermal expansion coefficients and water absorption coefficients of the light guiding plate 1 and the reflecting sheet 3 differ from each other. Therefore, when the light guiding plate 1 and the reflecting sheet 3 expand or contract due to a variation in humidity and temperature in the vicinity of the lighting unit UT, a difference occurs between an amount of expansion or contraction of the light guiding plate 1 and that of the reflecting sheet 3, due to a difference in the thermal expansion coefficient or the water absorption coefficient between the plate 1 and the sheet 3. Since the reflecting sheet 3 is bonded to the bottom surface of the light guiding plate 1 by the adhesive as described above, there is a possibility that deflection occurs in the reflecting sheet 3, when the difference occurs between the amount of expansion or contraction of the reflecting sheet 3 and that of the light guiding plate 1. Since the deflection of the reflecting sheet 3 is reflected in the light emitting surface of the lighting unit UT as non-uniform luminance, there is a possibility that this negatively affects uniformity of the illuminating light of the lighting unit UT.
Also, in the above-described structure, since the thermal expansion coefficients and the water absorption coefficients of the reflecting sheet 3 and the electrically conductive sheet 5 differ from each other, a difference occurs between the amount of expansion or contraction of the reflecting sheet 3 and that of the electrically conductive sheet 5, due to a difference in the thermal expansion coefficient and the absorption coefficient between the sheet 3 and the sheet 5, as in the case of the light guiding plate 1 and the reflecting sheet 3. Since the electrically conductive sheet 5 is bonded and fixed to the surface of the reflecting sheet 3 corresponding to the reflector portion RF by the adhesive as described above, there is a possibility that deflection or crease occurs in the reflecting sheet 3 corresponding to the reflector portion RF to which the electrically conductive sheet 5 is bonded, when the difference occurs between the amount of expansion or contraction of the reflecting sheet 3 and that of the electrically conductive sheet 5. Since deflection or crease of the reflecting sheet 3 is reflected in the light emitting surface of the lighting unit UT as the non-uniform luminance, there is a possibility that these negatively affect the uniformity of the illuminating light of the lighting unit UT.
Especially, in a region K1 on the rear surface side of the light guiding plate 1 corresponding to the reflector portion RF, occurrence of slight deflection or crease makes the luminance vary locally in a region with the deflection or the crease, thereby causing the luminance of the region to increase drastically. As a result, the non-uniform luminance is increased. If the electrically conductive sheet 5 is not disposed in the region K1, then occurrence of deflection or crease of the reflecting sheet 3 are inhibited. But in this case, the electromagnetic wave radiated from the fluorescent discharge tube 2 reaches the circuit board 8 and the like through the region K1, and adversely affects them. Therefore, it is necessary to dispose the electrically conductive sheet 5 in the region K1. On the other hand, in a region K2 on the front surface side of the light guiding plate 1 corresponding to the reflector portion RF, since the reflecting sheet 3 is bonded and fixed to the light guiding plate 1 by the adhesive 7a, deflection or crease hardly occurs in the reflecting sheet 3. And, in a region on the end face side of the light guiding plate 1 corresponding to the reflector portion RF, since the reflecting sheet 3 is disposed outside of the light guiding plate 1, the influence on uniform luminance of the light emitting surface of the lighting unit UT is not large even if deflection or crease occurs in the reflecting sheet 3.
Besides occurrence of the above-described non-uniform luminance, there is also a problem that heat of the fluorescent discharge tube 2 is deprived by the electrically conductive sheet 5 when the sheet 5 is disposed on the reflector portion RF. So, luminance rising characteristic of the lighting unit UT is deteriorated when the lighting unit UT starts to be lighted under low-temperature environment, as compared to the case where the sheet 5 is not disposed.
In the meantime, such non-uniform luminance due to deflection or crease of the reflecting sheet 3 is considered to be solved by forming the entire or a part of the reflecting sheet 3 by a solid material such as a metal plate having a reflective surface. However, when the reflecting sheet 3 is comprised of the metal plate, thickness and weight thereof increase, thereby increasing thickness and weight of the entire lighting unit UT as compared to the above-described reflecting sheet 3 comprised of the resinous film. Furthermore, a cost of the sheet also disadvantageously increases. In order to inhibit occurrence of deflection or crease of the reflecting sheet 3 in the region K1 on the rear surface side of the light guiding plate 1 corresponding to the reflector portion RF, it is conceived that the light guiding plate 1 and the reflecting sheet 3 are bonded to each other by an adhesive such as the double face adhesive tape, as in the case of the region K2. In this case, however, the luminance in the region K1 extremely increases due to reflection by the adhesive, thereby causing the non-uniform luminance to adversely occur.